1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus that can be used for a copying machine, a printer, and so on.
2. Description of the Related Art
Two major types of conventional full color electrophotographic image forming apparatus include the so-called four-cycle type and the so-called tandem type. The electrophotographic image forming apparatus of the four-cycle type has a sensitized body and a developing unit that contains toners of a given number of different colors. As the sensitized body is driven to rotate, toner images of different colors are sequentially superposed to form on the sensitized body and transferred onto an object of transfer, one upon another to produce a color image. On the other hand, the tandem type has a given number of sensitized bodies that are provided with respective developing devices of different colors. A color image is produced on an object of transfer as the toner images formed on the respective sensitized bodies are sequentially superposed and transferred onto it.
Since the sensitized body and other related parts of the four-cycle type apparatus need to be rotated for the given number of cycles (typically four cycles) to produce a color image, the apparatus is accompanied by a problem of a low image forming rate. Therefore, the tandem type apparatus is preferentially used when images need to be formed at a high rate.
FIG. 1 of the accompanying drawings schematically illustrates an image forming apparatus having four photosensitized drums arranged in series and an intermediate transferring belt.
Referring to FIG. 1, the apparatus has four photosensitized drums 11Y, 11M, 11C and 11K arranged in series, on which respective toner images of Y (yellow), M (magenta), C (cyan) and K (black) are formed, an endless intermediate transferring belt 20 which is held by a number of rolls including a drive roll 22 for driving the belt 20, a tension/steering roll 23 for correcting the meandering movement, if any, of the belt 20, a backup roll 24, a tension roll 25 and a follower roll 26 and adapted to move while being held in contact with the four photosensitized drums 11Y, 11M, 11C and 11K arranged in series, primary transferring rolls 27Y, 27M, 27C and 27K arranged respectively opposite to the photosensitized drums 11 with the intermediate transferring belt 20 nipped between them to transfer the respective toner images onto the intermediate transferring belt 20 and a secondary transferring roll 28 arranged opposite to a backup roll with the intermediate transferring belt 20 nipped between them to transfer the toner images carried by the intermediate transferring belt 20 onto a sheet of paper P. The image forming apparatus additionally has a reference mark sensor 31 for detecting that the intermediate transferring belt 20 has just made a full turn and an edge sensor 32 for detecting the edges of the intermediate transferring belt 20. As viewed in the moving direction B of the intermediate transferring belt 20, the reference mark sensor 31 is arranged upstream relative to the photosensitized drum 11Y for forming a Y-color toner image whereas the edge sensor 32 is arranged downstream relative to the photosensitized drum 11K for forming a K-color toner image.
Since the tandem type image forming apparatus is adapted to drive all the given number of sensitized bodies to rotate simultaneously, unnecessary sensitized bodies have to be driven when the black and white mode of operation is selected, while all the sensitized bodies are driven to rotate as a matter of course when the color mode of operation is selected. Thus, the sensitized bodies are inevitably worn to a large extent. Additionally, if the sensitized bodies that do not participate in the operation of forming an image are held in contact with an object of transfer, the developing toners remaining on the sensitized bodies are wasted.
To dissolve this problem, Japanese Patent Application Laid-Open Publication No. 2001-242680 proposes an image forming apparatus having a belt-shaped member (an intermediate transferring member, a sensitized body, a paper-conveying member, and so on) and plural juxtaposed oppositely disposed members (sensitized bodies, developing agent carriers and so on) disposed opposite to the belt-shaped member as well as means for separating the belt-shaped member and some of the oppositely disposed members in order to separate unnecessary oppositely disposed members from the belt-shaped member.
The proposed image forming apparatus is believed to be able to avoid the problem of unnecessary wear of the oppositely disposed members.
However, since the separating means is arranged outside relative to one end of the row of the juxtaposed oppositely disposed members and forced to swing up and down in order to separate some of the oppositely disposed members from the belt-shaped member, the image forming apparatus inevitably has large dimensions.
On the other hand, if the profile of the transferring belt is altered for the purpose of separating unnecessary sensitized bodies from the transferring belt, it becomes highly difficult to accurately control the tensile force of the belt. Then, there arises a problem of a meandering belt.
Japanese Patent Application Laid-Open Publication No. 2000-181184 discloses an image forming apparatus having at least a pair of sensitized bodies, a sheet of paper conveying means and a transferring means and adapted to prevent any meandering movement of the belt and unnecessary wear of the sensitized bodies from taking place, by moving both the conveying means and the transferring means with respect to the axis of rotation of one of the sensitized bodies in order to separate the contact point of the sensitized body and the conveying means from the other sensitized body.
However, image forming apparatuses disclosed in Japanese Patent Application Laid-Open Publication Nos. 2001-242680 and 2000-181184 are still accompanied by a problem that they need a complex mechanism for separating the sensitized bodies adapted to carry respective images formed thereon from the belt.